Manufacture of electrical rectifiers



Sept. 7, 1943. P. H. BOWLING MNUFACTURE OF ELECTRICALRECTIFIERS Filed Aug. 19, 1959 'zb-glu!! Maig/zer 7012061A Copper ry. @am w d @if 0,5 Ar a@ 0M H15 ATTORNEY Patented Sept. 7, 1943 MANUFACTUBE OF ELECTRICAL RECTIFIEBS Philip n. Dowling, Forest mus, ra., assigner to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application August 19, 1939, Serial No. 291,052 7 Claims. (Cl. 2011-15) My invention relates to the manufacture of electrical rectiers, and particularly to the manufacture of electrical rectilers of the well-known copper oxide variety.

One object of my invention is to provide novel and improved method for reducing the outer surface of the copper oxide layer of a copper oxide rectiiier element to metallic copper to facilitate making electrical contact with such oxide surface.

Another object of my invention is to provide an improved method for producing a good metallic contact with the cuprous oxide layer of a copper oxide rectier by rst electrolytically 'reducing the outer surface of the copper oxide layer to metallic copper, and then plating nickel onto the metallic copper so formed.

Other objects and characteristic features of my invention will appear as the description proceeds.

The process disclosed herein is an improvement uponA that described and claimed in the copending application forLetters Patentof the United States, Serial No. 213,876, filed by myself and John D. McCluer on June 15, 1938, for Electrical rectiers, now Patent No. 2,197,632, granted April 16. 1940.

The present application is a continuation-in I part of my prior application, Serial N o. 225,369, led August 17, 1938, for Electrical rectiiiers, now abandoned.

I shall describe one form of rectifier embodying my invention, and one process for rectifier manufacture also embodying my invention, and shall then point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 1s a view showing in elevation one form of blank ready to be prepared as a rectier element in accordance with one process of manufacture embodying my invention. Fig. 2 is a view showing a number of blanks assembled on a support as they appear during one step in the process of manufacture. Fig. 3 is a vertical sectional view showing, in exaggerated form, a rectifier element as it appears in .another step in the process of manufacture embodying my invention. Fig. 4 is a side view show- 'ing a rectier element as it appears in still another step in the process of manufacture embodying my invention. Fig. 5 is a longitudinal sectional view of an electrolytic bath utilized in the proc.`

ess of -manufacture embodying my invention. Fig. 6 is a side view of a masking device illustrated in connection with thebath shown in Fig. 5 for, a purpose which will'be made'clear in the following specication. Fig.' 'l is a vertical sectional view showing a rectifier element as it appears in a.further step in the process of manufacture embodying my invention. Fig. 8 is a vertical sectional view-similar to Figs. 3, 4, and 7,

showing a completed rectifier element construct- -ed in accordance with my invention. Fig. 9 is a diagrammatic view showing a circuit arrange.-

ment which may be' used during one step in the process of manufacture embodying my invention.

Similar reference characters refer to similar g parts in each of the several views.

Referring to the drawing, the reference character A designates a blank of suitable material such as copper. As here shown, this blank is of circular conguration, and is provided with a central aperture i, althoughthis particular form is not essential to my invention. A number of -these blanks are assembled on a suitable support B in pairs in the manner shown in Fig. 2, so that the blanks of each pair have their adjacent faces Ai in contact, and the blanks are then subjected to an oxidizing process such, for example, as the application of heat in an oxidizing atmosphere to form an oxide coating on the blanks.

Thev oxidizing process is continued until a suiiicient deposit of oxide is formed over the exposed surfaces of the blanks. Each blank will then appear as shown in Fig. 3, from which it will be seen that the blank A is now covered with an inner coating D of cuprous or red oxide of copper and a thin outer coating C of cupric or black oxide of copper. It will also be seen from an inspection of Fig. 3 that the nat face of the blank which is exposed during the oxidizing process, that is to say, the lower face, has a much heavier oxide coating than the upper face. After the oxidation is completed, the elements are cooled in any suitable manner as by quenching them in water, and the elements lare then treated to remove the cupric oxide layer from the Ventire surface of the blanks, and to remoye the cuprous oxide coating from only one face' Al of l the blanks. This treatment may take a variety of forms, but will preferably consist in subjecting the blanks to a solution consisting of 2% by volume of sulphur-ic acid, and .1% by volume of hydrochloric acid in water. The temperature at which the solution is used is not critical, but the desired action of the solution is materially speeded up if the solution is hot, preferably at a temperature of to 100 C. The elements are preferably immersed in the solution for a time interval which is approximately twice as long as is required to remove the cupric oxide. After the elements are immersedin the solution for the desired period of time, the elements are then removed from the solution, washed in water, and thoroughly dried.

'Ihe elements are next preferably immersed are left ou the elements from the water, the final rinse water should bedistilled water unless-the available water is known to be free from injurious impurities. The immersion in the nitric acid removes any loose or finely divided copper left on the blanks by the treatment of the sulphuric and hydrochloric acid solution and insures that the copper oxide surface will be sufiiciently clean for the next step which should preferably be performed before the elements have had any chance to accumulate any foreign substances either from the atmosphere or otherwise. The elements following this step will appear as illustrated in Fig. 4 in which D designates'the coating of cuprous oxide remaining on the inner and outer edges of the blank and on one face A2 of the blank.

The portion of the process of manufacture of the rectifier elements thus far described is well known, and for a further description of this portion of the process reference may be had to Letters Patent of the Uni-ted States No. 2,094,642, granted to me on October 5, V193'?, for the Manufacture of electrical rectiers.

The elements are next treated to reduce the outer surface of the cuprous oxide coating of the blanks to metallic copper. This reduction may .be accomplished in a variety of ways, but it is essential that the means employed for reducing the copper should not impair the rectifying properties of the elements, and it is also essential Athat the resultant copper coating should adhere firmly to the oxide coating and should make good electrical connection with it. Itis further essential that the surface of the oxide at the coppercuprous oxide junctions should not be reduced because if it were reduced the elements would becomeshort-circuited and would therefore be rendered useless for their intended purpose.

In accordance with my present invention, I accomplish the desired reduction by immersing the elements in an electrolytic bath comprising a dilute solution of ammonium hydroxide and passing a current through this ba-th to the elements from an insoluble anode such as platinum or carbon, and in Figs. 5 and ,6 I have shown a pair of elements A immersed as the cathode in the electrolytic bath, the bath being contained in a tank 2, and the elements being mounted in a device C which serves both as a. means for making electrical contact with the mother copper of the elements, and also as a means for masking the outside and inside edges of the elements to prevent the copper oxide from being reduced adjacent the edges where it joins the mother metal. l

'I'he device C may assume a variety of forms, and

- as here shown, comprises a metal washer 3, such for, exampleas brass cemented, as by a latex compound, between two rubber washers 4, and connected to a terminal wire 5. The rubber washers l have substantially the same thicknesses as the. elements A following the previously described acid treatment, and substantially the same inside diameters as the outside diameters ci' the elements, and the elements are disposed within the openingsin these washers with their faces, from which the oxide has been removed, in contact with the metal washer 3. The outside edges of the elements are masked by means of a pair of rubber washers 6 having inside diameters that are slightly smaller than the outside diameters of the elements, and which washers are clamped against the outer edges of the oxidized faces of the elements i-n concentric relation thereto by means of insulated washers 'l and bolts 8, whereby portions' ci' the washers 6 overlap the oxidized faces of the elements adjacent their outer edges and serve to substantially seal the oxide surfaces against theaction of the electrolyte for a small annular area inside of the adjacent coppercuprous oxide junction. The washers yli are preferably each provided at their upper edges with one or more vents 6a to allow the escape of gases. 'I'he inside edges of the elements are masked by means of a pair of rubber washers 9 having outside diameters which are slightly larger than the diameters of the holes in the elements, and which Washers are clamped against the inside edges of the elements in concentric relation to the elements by means of a pair of insulating clamping members l0 and Il in such manner as to substantially seal the copper oxide surfaces of the elements against the action of the electrolyte for a small annular area adjacent the inner copper-cuprous oxide junction An anode is preferably provided for each element, and, as shown in Fig. 5, each of these anodes consists of an insoluble electrode I3 such asplatinum to which external connections are made by a conductor Il. To accomplish the desired reduction. current is of course passed from the electrodes i3 to the rectifier elements,

extremely adherent, and disposed as a continuous film over the entire exposed face of the element.

A rectifier element manufactured in accordance with the process thus far described ages as well as, or better than, the well-known elements in which contact is made to the copper oxide surface by coating this surface with graphite and then pressing an impressionable electrode such as lead into contact with the graphite coated surface under relatively heavy pressures, and, in addition, has the advantage that it possesses a better lateral conductivity so that a high assembly pressure is not necessary. In fact, an element constructed in the manner just described maybe used where contact is made only over limited areas of the reduced surface. Furthermore, by prolonging the reduction for a length oi' time which is sufcient to materially reduce the thickness of the oxide coating, a lower forward resistance can be obtained than can be obtained by the use of a graphite-lead content. The exact reasons for this reduction in the forward resistance are Very difficult t0 determine, but this reduction would appear to be due to one or more thereby providing a contact' area between the area" of 2 square inches.

oxide and the copper layer which may have many times the plane surface area of the oxide layer.

This latter effect seems to be very important because the reduced copper formed on the copper oxide appears to have a wallie-like surface, and the vresulting lgrid need only penetrate into the oxide .001 inch or less in order to produce a contact area several times that of the plane area that would be provided if' no penetration took place. For example, if it be assumed that the mesh fof the grid formed by the reduced copper is .002 inch square and penetrates the oxide to that the reduced copper does not have to penetrate very far between the crystals of the copper oxide before a rather large increase in contact area is achieved.' 'I'he resistance per unit area between the copper Oxide and the reduced copper is sufdciently high so that this increase in contact area will be reected in a-considerable decrease in the overall resistance of the rectifier element as measured in the low resistance direction. In fact, by prolonging the reduction the forward resistance at, the usual working voltages can be decreased as much as 50% over the lowest resistance that can be obtained by theolder methods of securing contact with the oxide. (3) A decrease in the resistance per unit area of the contact between the oxide and the copper formed by the reduction.

The amount .of the reduction which takes place depends to some extent upon the temperature of the reducing solution, the rate of reduction being increased withincreased temperatures. If the temperature becomes too high, the reduction may be carried into the oxide too far, and a short-circuited element will result. The ammoniacal solution specified has a relatively high resistance which necessitates a relatively high voltage, and tends to cause heating of the solution by the reducing current. Lowerconcentrations of the ammonium hydroxide can be used, but, the voltage `required to eiect the desired reduction increases as the concentration de-` creases. To facilitate temperature control it is desirable that thereducing solution be of a relatively low resistance, and this result may be obtained either by using a higher concentration of NHiOH, or by adding a small quantity of a soluble neutral salt to the above specied solution. For example, if .05% by volume of a saturated C.) solution of (NH-02504 is added to the above solution, the voltage required to accomplish the reduction will be decreased Excellent results can also be obtained by using a very dilute .ammoniacal solution ofammonium carbonate. This latter solution can be made by passing gaseous CO2l through a solution of NH4OH in distilled water.. The concentration of the NH4OH. should be very low, i. e., in the neigh.

borhood of .2% by volume of the usual concentrated NHiOlEIv (28.5% NH3, sp. grav.=.9) or about .05% NH3 by weight. The concentration of the CO2 in the solution is not critical, but if the CO: is added until the specific resistance `ofl the solution at 25 C. is inthe vicinity of 850 ohm-cm., excellent results are obtained. When this latter solution is used. there is a tendency to produce a streaked'copper surface unless the solution s-agitated. This may be accomplished by moving slightly the element whose copper surface is being reduced. For a 1%" element, a motion of V4" parallel to the face of the element at the rate of one or Atwo cycles per second is suflicient.

As has been pointed out hereinbefore, if the reduction is carried on for too great a length of time, there is a danger or short-circuiting the rectifier element due to the copper formed by the reduction penetrating to the mother copper. The allowable amount of the reduction depends upon the original thickness of the oxide which,'in turn, depends principally upon the temperature and length of time of the original oxidation. For example, an element oxidized for 13 minutes at 1860 F. can'be reduced for about 8 minutes at about 12 amperes per square foot in a bath of about 25 C. without causing any diiilculty due to the copper penetrating too deeply into the oxide .and thus short-circuiting the elements or causing too low a resistance in the current blocking or high resistance direction. Similarly, an element oxidized for 50 minutes at 18 60 F. can be reduced for 25 minutes under the conditions outlined above, and will have about the same electrical characteristics as the element which was oxidized.

' for only 13 minutes and reduced for 8 minutes.

In the case of the element which is oxidized for 13 minutes and then reduced for s-minutes, the

temperature of the bath should not exceed 30 C.

When the reduction ofthe copper oxide surface o f the elements is prolonged to decrease the forward resistance of the elements, it is necessary in order to avoid danger of short-circuiting the elements that the reduction should be uniform over the entire portion of the surface which is being reduced. As there is naturally a tendency for the current-to concentrate at the edges of the elements.' the elements should be shielded, or

, some device should be employed which will reduce the concentration at the edges. One convenient means for accomplishing the necessary 'shielding consists in making the washers 1 Ywhich clamp the maskingwashers 6 against the outer` edges of the elements with an inside diameter of such size that the inner edges of the washers 1 will project past the inner edges of the washers B for some ydistance in the manner shown in Fig. 5.

'Upon removing the elements from the electrolytic reduction bath,the elements are thoroughly rinsed in-several rinse waters, the last rinse water preferably being distilled water, and the elements are then. subjected to a vigorous blast of compressed air. The air blast appears to have the effect of blowing out some of the gas and liquid which collects in the pores of the reduced copper, and seems to be helpful in preparing the elements for the next step which I shall describe presently.

l It should be pointed out that a rectifier element constructed in the manner thus far described is superior to rectifier elements in which the oxide coating'is reduced to copper in the' manner described in the previously referred to Dowling and McCluer application in that the copper formed by the reduction is brighter and tar;

nishes less readily, and that the elements age" better. 1t 1s thought that the improvement in the characteristics of the elements is obtained because of the volatile nature of the electrolyte which insures that no traces of the electrolyte will remain on the elements to later cause corrosion or otherwise aiect the characteristics of the reduced surface.

While rectifier elements constructed in the manner thus far described are satisfactory for many applications without further treatment, when the elements are to be used in applications where moisture is present, the aging characteristics of the elements may be improved by depositing a layer of nickel on the reduced copper layer. To accomplish this step, the elements while still mounted in support C are made the cathode in a nickel plating solution in the same manner that they were made the cathode in the ammonium hydroxide bath during the preceding step.

The proper technique for accomplishing the nickel plating in the nickel plating solution has been found to depend upon the amount that the copper oxide has previously been reduced to metallic copper, and even with'the same amount of reduction, upon the particular reducing solution used. The standard nickel plating solution described in the previously referred to Dowling and McCluer application may be used, but I prefer to use a more concentrated solution, which may, for example, have the following composition; NiSO4.7H2O, 27 ounces per gallon; boric acid, 6 ounces per gallon; NiClzl-IzO, 4 ounces per gallon. This latter solution is well known, but appears to be better adapted for the plating of the reduced copper surface than the solution referred to in the previously described Dowling and McCluer application. y

When the solution just described is used, I have found that in general there are three stages involved in applying the nickel to the reduced copper surface, as follows:

First stage During this stage there is but relatively little change in the color of the copper surface with but little visual evidence of the deposition of bulk nickel. The duration of this stage may be a large fraction oi the total plating time. It depends upon many factors and can be found for any particular set of conditions, only by trial. Throughout this rst stage the forward resistance of the elements remains practically unchanged.

Second stage immediately after the previously described re` duction of the oxide surface of the copper oxide to copper was completed. This stabilization can be accelerated by the application of heat. A heat treatment of 1 hour at 80 C. has been found to be satisfactory. Other temperatures and times may, however, be used, the necessary time decreasing as the temperature increases. Tem;

peratures higher than 125 C. 4should not be employed, and, in general, the minimum time for stabilization should not be greatly "exceeded so that the volume resistivity o f the oxide will not be unduly increased. However, it has heretofore been customary to preage the rectifier elements for certain applications by a thermal treatment consisting of approximately a 16 to a 24 hour bake at C., and when this last mentioned treatment is employed, the stabilizing heat treatment just described may be eliminated.

Third stage reduced copper surface formed by the hereindescribed reducing process depends upon so con-` trolling the current density, acidity and temperature of the plating solution, and plating time that if the plating is done in one continuous operation, the second stage will be reached but not passed. If, however, by plating in a continuous operation it is impossible to obtain the thickness of nickel which is necessary to give suilicient lateral conductivity for certain applications of rectiers, without causing the forward resistance of the elements to increase, then the plating can be divided into two operations, as will be made clear presently. l 1

The current density employed during the plating step, and the length of plating time that can be employed, dpends upon the original amount of the reduction, the pH value of the plating solution, and the degree of agitation of the soluelements into the second stage, and there is also a tendency for the allowable current density to increase as the amount of the reduction is increased. For example, an element the -coppper oxide surface of which has been reduced to copper for 8 minutes under the same conditions as an element which has been reduced vfor 3 minutes, will be plated into the second stage in the preferred nickel plating solution by using a current density of 24 amperes per square foot for 2 minutes. Other current densities and times may be used, but this combination gives excellent results. 'I'his combination will not, however, carry vinto the second stage an element whose copper For larger elements or batches of elements, a higher pH value, in the neighborhood of 5.8-6.0 with agitation throughout the plating operation appears to be desirable. It is desirable to keep the solution clear and free from suspended matte'r by filtering when necessary. While the current density and plating times speciiied are not very critical, too low or too high a current density and too short or too long a plating time tend to produce either two high forward resistances or rectiers which will not stand unprotected-exposure to moisture very well.

When the elements are plated into the second stage in a single operation, it appears that a thorough drying of the elements following their removal from the reducing solution, as described hereinbefore, is beneficial in postponing the onset of the third stage. It has been found, however, that this drying is not essential and that good results can be obtained by merely rinsing the elements in clear water between the reducing and the plating steps. A quick agitation of the elements as they enter the nickel plating. solution is desirable when the elements have not been thoroughly dried following their removal from the reducing solution, to insure a quick distribution of substantially undiluted nickel solution over the entire surface which is to be plated.

It is thought that the conditions obtaining at the surface of the element, at the moment that the nickel plating commences, rather critically -de termines the subsequent course of the plating, and the final quality of the electrical contact to the copper oxide. that if the reduced copper surface is allowed to come into contact-with the nickel plating solution for even a few seconds withno voltage ap- For example, I have found power supply will be higher than usual. By this plied, the contact is likely to be ruined, and the forward resistance of the elements is likely to be rendered permanently high. It is obvious, therefore, that the plating voltage should be applied to the electrodes before the elements are placed in the nickel plating solution.

I have also found that when a rectifier element is to be plated on both sides, or when two elements are each adapted to be simultaneously plated on one side by means of an arrangement similar to that shown in Figs. 5 and 6, if there are any diierences in the two surfaces which are to be' plated, these diiferences may become i`ncreased during .the plating operation until the current passing to the one surface is several times that passing to the other surface, and that no amount of change in the position of the elements in the bath relative to the .anodes will restore the balance. 'I'his difference in current ow may cause one surface to receive mostl of the nickel,

' or if the unbalance is not sufficient to have this effect, one surface may be carried into the third stage of plating, with a permanently high forward resistance, while the other surface will only be carried into the second stage and .will have a 'forward resistance which will stabilize at an acceptable value.

Furthermore, when several elements are to be simultaneously plated at different locations in the nickel plating bath, if there are differences in the oxide and rectifying Junction resistances of the individual elements and/or in the resistances of the contacts to the copper of the individual elements, unequal current distribution among the various elements will be produced with consequent erratic results in the plating.

In order to avoid these difficulties, I prefer to` utilize for the nickel plating bath circuits which tribution to both sides of a numberof elements which are all to be plated in the same bath. Each element E` is positioned between its individual pair of electrodes I3. All of the anodes on one side of the elements are connected to'an anode bar I5, each through its individual fixed resist-v ance R of relatively high impedance. All of the anodes on the opposite sides of these elements are similarly connected to another anode bar I6. The mother copper of each element is connected .to a common cathode bar I1, it being pointed out thatvwith this arrangement both sides of eachv element will have been oxidized and will have had both oxide surfaces subjected to a reducing treatment similar to that described hereinbefore. The total current to the two anode bars I5 and, I6 is controlled and is brought to the same valuevby means'of two resistors RI and R2, suitable ammeters XI being provided to facilitate the necessary adjustment. In order to minimize the amount of current which can reach any element from a neighboring anode, the separation between adjacent anodes should be large compared to the separation between each anode and its associated element. Preferably insulating shields IB will be provided between adjacent elements.

As has been pointed out hereinbefore, once a rectier element has been given a chance to stabilize its forward resistance afterbeing plated into the second stage, the plating may then be continued without carrying it into the third stage. As an example, an element whose copper oxide surface has been reduced to metallic copper for 8 minutes, and has then been plated into the second stage in the manner previously described, can, if it is allowed to stabilize its resistance, be nickel plated for an additional 2 minutes at 24 amperes per square foot with but little effect on the forward resistance, even when measured immediately after the second plating. Stabilization in this case can be completed in about 16 hours at room temperature and ordinary atmospheric conditions. Stabilization which will allow suillcient subsequent plating is not necessarily indicated by-the return, plating, of the forward resistance to its original value. The original value of the 'forward resistance is usually reached before the element is ready for its second plating. There is no visible change accompanying the complete stabilization. and its presence for a given combination of reduction and plating can be ascertained only by trial.

After the nickel plating meteen completed,

the elements should be thoroughly rinsed in clean they may be after the first In cases where greater lateral conductivity is required than that obtained by the reducedvcopl per-nicke1 contact described above, as for example might be the case in connection with elements which are to be used in electroplating applications, either the reduced oxide contact alone may be sprayed with a low melting point metal, or this low melting point metal may be sprayed over the nickel plated reduced copper contact. The spraying can be done by well-known processes, such, for example, as the Shoop process.

One advantage of a rectiiier element constructed in accordance with my invention is that the characteristics of the elements obtained are 1. 'I'he process of making Contact with the cuprous oxide surface of a copper oxide rectier element'which process consists in masking the edges of the oxide surface, making the element the cathode in an electrolytic bath comprising a dilute solution of ammonium hydroxide in water and passing a current through said bath to said element to reduce a. portion of the cuprous oxide surface to metallic copper, and then making the element the cathode in a nickel plating solution consisting of 27 ounces per gallon of NiSOrHI-IzO, 6 ounces per gallon of boric acid, 4 ounces per gallon of NiCl2.6I-I2O and passino' a current of about 12 amperes per square :met f..`.'= i-ough said plating solution to said element for about 3 minutes.

2. 'I'he process of making contact with the cuprous oxide surface of a copper oxide rectifier' element which process consists in masking the edges of the oxide surface, making the element the cathode in an electrolytc bath comprising a dilute solution of ammonium hydroxide in Water and passing a current through said bath to said element to reduce a portion of the cuprous oxide surface to metallic copper, and then making the element the cathode in aV nickel plating solution consisting of 27 ounces per gallon oi NiSO4J7mO, 6 ounces per gallon of boric acid, 4 lounces per gallon of NiClrH-iO and passing a current of about 12 amperes per square foot through said plating solution to said element for about 3 minutes, said solution being maintained at a pH value of 5.0 to 5.2, and finally applying heat to said element to stabilize the forward resistance of the element.

3: The process of making contact with the cuprous oxide surface of acopper oxide rectifier element which process consists in masking the edges of the oxide surface, making the element the cathode in an electrolytic bath comprising a dilute solution of ammonium hydroxide in water and passing a current through said bath to said element to reduce a portion of the cuprous oxide surface to metallic copper, and then making the element the cathode in a. nickel plating solution consisting of 27 ounces per gallon of NiSOiHHrO, 6 ounces per gallon of boric acid, 4 ounces per gallon of NiC1r.6H.'O and passing a current of about 12 amperes per square foot through said plating solution to said element for about three minutes, said solution being maintained at a pH value of 5.0 to 5.2. and finally subjecting said element to a temperature of approximately C. for an hour.

4. 'Ihe process of making contact with the cuprous oxide surface of a copper oxide rectiier element which process consists in masking th'e edges of the oxide surface, making the element the cathode in an electrolytic bath comprising a dilute solution of ammonium hydroxide in water and passing a. current through said bath to said element to reduce a portion of the cuprous oxide surface to metallic copper, and then making the element the cathode in a nickel plating solution consisting of 27 ounces per gallon'of NiSOiJlHzO, 6 ounces per gallon of boric acid, 4 ounces per gallon of NiClzHzO and pas/sing a current of about l2 amperes per square foot through said plating solution to said elementA for about 3 minutes, said solution being maintained at a. pH value of 5.0 to 5.2, and nally subjecting said element to a temperature of between 80 C. and 125 C. for a, period of time to stabilize the forward resistance of the element.

5. The process of electrolytically reducing a portion of the copper oxide surface of a. copper oxide rectifier element to metallic copper to iacilitate making contact with said surface which process consists in making the element the cathode in an electrolytic bath comprising' approximately .05% NH3 by Weight of concentrated NHiOH in water combined with a very dilute ammoniacal solution of ammonium carbonate, and passinga current through said bath to said element.

6. The process of electrolytically reducing a portion of the .copper oxide surf-ace of a. copper oxide rectiiier elementvto metallic copper to facilitate making contact with said surface which process consists in making the element the cathode in an electrolytic bath comprising approximately .05% NH.; by weight of concentrated NHiOH in water combined with a very dilute ainmoniacal solution of ammonium carbonate, and passing a current of the order of 12 amperes per square foot through said bath for approximately 8 minutes.

v7. The process of electrolytically reducing a portion of the copper oxide surface of a copper oxide rectifier element to metallic copper to fa.- cilitate making contact with.said surface which process consists in making the element the cathode in an electrolytic bath comprising approximately .05% NH3 by weight of concentrated NHAOH in water combined with a very dilute arnmoniacal solution of vammonium carbonate, and passing a current of the order of 12 amperes per square foot through said bath for approximately 8 minuten-the temperature of the bath being maintained at approximately 25 C.

-PIIELJP H. DOWLING. 

